This compilation error is encountered when you try to instantiate a struct that contains a mapping as an attribute. I needed this data structure to write to allow patients to license the datasets resulting from their clinical trials, as part of an grant. this contract Algovera This type of one-to-many data structure can be useful, for instance, to encode a marketplace for time-limited licenses to some content. In addition to the content to be licensed, the struct would need to map licensees' addresses with a timestamp for the start of their license, so that their access can be controlled. struct content{ address payable owner; bytes32 hash; mapping(address => uint) licenses; } Visually it would look like this: The "struct containing a (nested) mapping cannot be constructed" error would appear when trying to instantiate the struct, the usual way, with a recent compiler version. Like this, for instance: function submitContent(bytes32 _hash) public payable returns(uint _id) { _id=contents[msg.sender].length; contents[msg.sender].push(content({ owner: payable(msg.sender), hash:_hash return(_id); })); } Indeed, since version 0.7.0, structs or arrays that contain a mapping can only be used in storage, so Solidity complains because variables in an instantiation function would be in memory by default. You can read about this change in the . Solidity documentaiton The relevant part is under "Removal of Unused or Unsafe Features": I wanted to write this short note because some top results when Googling this error are either outdated, or to the extent of "just don't use a mapping in a struct bro". However, this type of structure cannot be equivalently modeled with a separate mapping for licenses; reading and writing would require much more computation. The workaround is quite simple, we just have to declare the struct in storage before we instantiate a pointer to it. Our function would have to look like this: submitContent() function submitContent(bytes32 _hash) public payable returns(uint _id) { _id= contents[msg.sender].length; content[] storage c = contents[msg.sender]; c.push(); c[_id].owner = payable(msg.sender); c[_id].hash=_hash; return _id; }

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How to Solve "Struct Containing a (Nested) Mapping Cannot be Constructed" in Solidity

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Nassim Dehouche

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How to Solve "Struct Containing a (Nested) Mapping Cannot be Constructed" in Solidity

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Nassim Dehouche

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